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# Can negating a non-negative signed integer value ever overflow?

Can negating a non-negative signed integer value ever overflow? Put another
way, can it be true that (mathematically) INT_MIN > -INT_MAX, LONG_MIN
-LONG_MAX etc?

I know that typically it can't overflow, but if that isn't guaranteed, how
can I portably detect if it would?

Alex
Mar 6 '06 #1
8 2429
On 2006-03-06, Alex Fraser <me@privacy.net> wrote:
Can negating a non-negative signed integer value ever overflow? Put another
way, can it be true that (mathematically) INT_MIN > -INT_MAX, LONG_MIN
-LONG_MAX etc?

I know that typically it can't overflow, but if that isn't guaranteed, how
can I portably detect if it would?

Alex

C99 only allows twos-complement, ones-complement, and signed-magnitude
representation. It is unclear what C89 allows.
Mar 6 '06 #2
Alex Fraser wrote:

Can negating a non-negative signed integer value ever overflow?

No.

--
pete
Mar 6 '06 #3
"Jordan Abel" <ra*******@gmail.com> wrote in message
news:sl***********************@random.yi.org...
On 2006-03-06, Alex Fraser <me@privacy.net> wrote:
Can negating a non-negative signed integer value ever overflow? Put
another way, can it be true that (mathematically) INT_MIN > -INT_MAX,
LONG_MIN > -LONG_MAX etc?
C99 only allows twos-complement, ones-complement, and signed-magnitude
representation.

Which implies INT_MIN is either -INT_MAX (one's complement or sign and
magnitude) or -INT_MAX - 1 (two's complement), etc, so that's OK.
It is unclear what C89 allows.

Hmm. A shame.

I know it normally can't overflow, and I can't see how to detect if it
could/would overflow anyway, so I guess should shrug my shoulders and move
on :).

Thanks,
Alex
Mar 7 '06 #4
pete wrote:
Alex Fraser wrote:
Can negating a non-negative signed integer value ever overflow?

No.

No context? Chuck is gonna getcha.

Assume 2's complement.

int i = INT_MIN;

This is -2147483648 and 0x80000000. If then..

i = -i;

...the result is the same. Did anything happen? Was it overflow?

--
Joe Wright
"Everything should be made as simple as possible, but not simpler."
--- Albert Einstein ---
Mar 7 '06 #5
Joe Wright said:
pete wrote:
Alex Fraser wrote:
Can negating a non-negative signed integer value ever overflow?

No.

No context? Chuck is gonna getcha.

Assume 2's complement.

int i = INT_MIN;

This is -2147483648 and 0x80000000. If then..

i = -i;

..the result is the same. Did anything happen? Was it overflow?

This may have escaped your notice, I know - but -2147483648 does not quite
qualify as non-negative, so it falls outside the domain of the question.

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at above domain (but drop the www, obviously)
Mar 7 '06 #6
Joe Wright <jo********@comcast.net> writes:
pete wrote:
Alex Fraser wrote:
Can negating a non-negative signed integer value ever overflow?

No.

No context? Chuck is gonna getcha.

What do you mean? pete quoted the relevant portion of the parent

--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.
Mar 7 '06 #7
Keith Thompson wrote:
Joe Wright <jo********@comcast.net> writes:
pete wrote:
Alex Fraser wrote:

Can negating a non-negative signed integer value ever overflow?
No.

No context? Chuck is gonna getcha.

What do you mean? pete quoted the relevant portion of the parent

Yes, I saw no reason to unleash the slavering dogs.

--
"If you want to post a followup via groups.google.com, don't use
the broken "Reply" link at the bottom of the article. Click on
"show options" at the top of the article, then click on the
Mar 8 '06 #8
CBFalconer wrote:
Keith Thompson wrote:
Joe Wright <jo********@comcast.net> writes:
pete wrote:

Alex Fraser wrote:
>Can negating a non-negative signed integer value ever overflow?

No.
No context? Chuck is gonna getcha.

What do you mean? pete quoted the relevant portion of the parent

Yes, I saw no reason to unleash the slavering dogs.

Sorry pete. Thanks Richard, Keith and Chuck for catching me up. I read
pete's one liner as..
"Can negating a negative signed integer value ever overflow?"

--
Joe Wright
"Everything should be made as simple as possible, but not simpler."
--- Albert Einstein ---
Mar 8 '06 #9

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